Hydrocarbons (such as oil, condensate, and gas) may be produced from wells that are drilled into formations containing them. For a variety of reasons, such as low permeability of the reservoirs or damage to the formation caused by drilling and completion of the well, or other reasons resulting in low conductivity of the hydrocarbons to the well, the flow of hydrocarbons into the well may be undesirably low. In this case, the well is “stimulated,” for example, using hydraulic fracturing, chemical (such as an acid) stimulation, or a combination of the two (often referred to as acid fracturing or fracture acidizing).
Hydraulic and acid fracturing treatments may include two stages. A first stage comprises pumping a viscous fluid, called a pad, that is typically free of proppants, into the formation at a rate and pressure high enough to break down the formation to create fracture(s) therein. In a subsequent second stage, a proppant-laden slurry is pumped into the formation in order to transport proppant into the fracture(s) created in the first stage. In “acid” fracturing, the second stage fluid may contain an acid or other chemical, such as a chelating agent, that can assist in dissolving part of the rock, causing irregular etching of the fracture face and removal of some of the mineral matter, which results in the fracture not completely closing when the pumping is stopped. Occasionally, hydraulic fracturing may be done without a highly viscosified fluid (such as water) to minimize the damage caused by polymers or the cost of other viscosifiers. After finishing pumping, the fracture closes onto the proppant, which keeps the fracture open for the formation fluid (e.g., hydrocarbons) to flow to the wellbore of the well. The performance characteristics of the proppant contribute to the overall effectiveness of the fracturing stimulation.
Proppant is typically made of materials such as sand, glass beads, ceramic beads, or other materials. Sand is used frequently as the proppant for fracture treatments. For fractures with high closure stress, such as greater than 6,000 pound per square inch (psi), in deep wells or wells with high formation forces, higher strength proppant is desired. The closure stress which sand can sustain is normally about 6,000 psi, so a closure stress over 6,000 psi would crush the sand into small pieces that reduces the width of the fracture and results in insufficient conductivity for oil and/or natural gas to flow. Furthermore, as the small pieces continually flow back during the production, the conductivity of the wells would reduce further which results in a short life span of the wells or results in refracturing having to be performed.
Ceramic proppant has been used to maintain the conductivity of wells with a high closure stress. Typically, the more the alumina (Al2O3) the proppant contains, the higher the closure stress the ceramic proppant can withstand, but also the higher the specific gravity of the proppant. A high specific gravity may lead to fairly rapid gravitational settling of the proppant, which results in difficulty to transport the proppant into the fracture, especially for fractures located far from the wellbore. Also, quick settling in the fracture leads to lack of proppant on the top part of a fracture, which reduces the productivity of the well.
High viscosity fracturing fluid, such as fluid containing a crosslinked polymer, may be used for transporting proppant with high specific gravity. However, fracture geometry, including width and height, is also affected by the fluid viscosity. High fluid viscosity leads to a large fracture width and may make the fracture excessively grow in height into a nonproductive or water-producing zone, impairing the efficiency of hydraulic fracturing.
To transport proppant of high specific gravity with fracturing fluid of a low viscosity, fiber has been added to the fluid as an additive. See, for example, U.S. Pat. No. 8,657,002, incorporated herein by reference in its entirety. To use fiber effectively for transporting proppant, the interaction force between fiber and proppant may have significance. For example, while a smooth surface and good sphericity are desired properties of a proppant particle in order to achieve high conductivity, these properties may result in a lower interaction force with fibers, which may require the use of a greater amount of fibers and, for stimulation techniques for geological formations that rely on proppant clusters/pillars to maintain the width of a fracture and channels for conducting the formation fluid, a lower interaction force between fiber and proppant may result in an increased tendency of spreading/collapse of the clusters under closure stress, which may reduce the channel size and/or eliminate channels. Retaining proppant surface smoothness and sphericity while achieving good interaction force between the proppant and the fiber is thus desirable.
The so-called drip-casting manufacturing technique has been adapted for the manufacture of spherical ceramic proppants. Drip-casting substitutes conventional ways of pelletizing (also called granulating) ceramic proppant such as using high intensity mixers and pan granulators. Vibration-induced dripping (or drip-casting) was first developed to produce nuclear fuel pellets. See U.S. Pat. No. 4,060,497. It has subsequently evolved into applications for metal and ceramic microspheres for grinding media, pharmaceuticals and food industry. An application of vibration-induced dripping to aluminum oxide spheres is described in U.S. Pat. No. 5,500,162. The production of the microspheres is achieved through vibration-provoked dripping of a chemical solution through a nozzle. The falling drops are surrounded by a reaction gas, which causes the droplet to gel prior to entering the reaction liquid (to further gel). Using a similar approach, U.S. Pat. No. 6,197,073 covers the production of aluminum oxide beads by flowing a sol or suspension of aluminum oxide through a vibrating nozzle plate to form droplets that are pre-solidified with gaseous ammonia before their drop into ammonia solution. U.S. Patent Application Publication No. 2006/0016598 describes the drip-casting to manufacture a high-strength, light-weight ceramic proppant. U.S. Pat. No. 8,883,693 describes the application of the drip-casting process to make ceramic proppant.
What is still desired, then, are ceramic proppants able to withstand high closure stress that have a low settling rate and high interaction force with fiber, while having a smooth surface and good sphericity.